1. Field of the Invention
The present invention relates to an image forming apparatus capable of performing both-sided printing, a method of controlling the same, a program for implementing the method, and a storage medium storing the program.
2. Description of the Related Art
Conventionally, there has been known a double-sided printing method using an image forming apparatus which prints one side of a printing sheet, then holds the semi-finished printing sheet with its one side having been printed on an intermediate tray, and inverts the sheet to print the other side thereof (a first example of double-sided printing). There has been known another double-sided printing method using an image forming apparatus which prints one side of a printing sheet, then conveys the sheet to a sheet inversion path to invert the sheet and print the other side thereof (a second example of double-sided printing).
The operation of an image forming apparatus according to the above-mentioned second example of both-sided printing will be described by referring to FIG. 2 showing an embodiment of the present invention for convenience. FIG. 2 is a schematic diagram showing a color image forming apparatus having four photosensitive drums. Upon start of both-sided printing, a sheet is picked up from a sheet cassette 221. The sheet is conveyed via the four photosensitive drums which are supplied with respective toners of colors YMCK (yellow, magenta, cyan, and black), so that one side of the sheet is printed, that is, a YMCK image is formed on one side of the sheet. The sheet with one side thereof having been printed is discharged once through a sheet discharge port to be switched back so that the sheet can be conveyed to a sheet inversion path. The sheet is then inverted, and a YMCK image is formed on the other side of the sheet. Thereafter, the sheet with both sides thereof having been printed is discharged through the sheet discharge port.
The second example of both-sided printing has the advantage that there is no necessity of providing an intermediate tray for receiving a semi-finished both-sided printing sheet as used in the first example of both-sided printing. In both the first example of both-sided printing and the second example of both-sided printing, there is the problem that a sheet is reconveyed even if there is only a print image to be printed on one side of a sheet as in the case where the total number of pages to be printed in a document is odd. Therefore, due to unnecessary sheet conveyance, the sheet jamming frequency is increased and the durability is deteriorated by wear of component parts used for sheet conveyance.
To address this problem, a printing apparatus disclosed in Japanese Laid-Open Patent Publication (Kokai) No. 9-254471, for example, is comprised of a page image determining means for determining whether a page includes an image to be printed, and a printing control means for providing control such that, in the case where the page image determining means determines that the page includes an image to be printed on only one side of a sheet, one-sided printing is performed without recirculating the sheet. With this arrangement, one-sided printing is performed on a sheet whose only one side is designated for printing, and this prevents unnecessary sheet conveyance, decreases the sheet jamming frequency, and prevents deterioration of the durability due to wear of component parts.
The above described prior art, however, has the following problem. According to the prior art, if it is determined that there is only a print image to be printed on one side of a sheet, such control is provided that one-sided printing is performed on the sheet, in any case. However, if this way of control is applied to prepunched sheets punched in advance for sheet binding and preprinted sheets on which images are printed in advance, a difference in printing direction can be caused by a both-sided printing mechanism. A brief description will now be given of this problem with reference to FIG. 7. Assuming that both-sided printing is performed on a five-page document (the first sheet (the first page and the second page), the second sheet (the third page and the fourth page), and the third sheet (the fifth page)), the third sheet is printed in a one-sided print sequence because, regarding the third sheet, there is a print image to be printed on only one side thereof (i.e. the fifth page).
In the printing operation, the first sheet is picked up first, and an image on the second page is printed on the first sheet. The first sheet is then conveyed to a sheet inversion path to be inverted, and an image on the first page is printed on the first sheet. The first sheet is then discharged to a sheet discharge tray. Similarly, an image on the fourth page and an image on the third page are printed on the second sheet in this order, and the second sheet is then discharged to the sheet discharge tray. When the third sheet is printed, the print sequence is switched from a both-sided print sequence to the one-sided print sequence, and an image on the fifth page is printed on the third sheet, which is then discharged to the sheet discharge tray. If the discharged three sheets are bound through binding holes, the printing direction of the fifth page is opposite to the printing direction of the other pages as shown on the right side of FIG. 7. This problem is also encountered in the case where preprinted sheets on which images are printed in advance are used.